This invention relates to a method and apparatus for regenerating contaminated acids through the formation of liquid hydrates. In particular, this invention relates to a method for purifying contaminated sulfuric and nitric acids, and mixtures thereof, by cooling the acid to a temperature sufficient to form a liquid hydrate of the acid.
Contaminated (or spent) acids streams are generated by a number of important industrial processes. For economic and environmental reasons, it is desirable to regenerate these contaminated acids so that they may be reused rather than discarded. One method of regenerating acids to remove contaminants is distillation. While generally effective, distillation methods require high temperatures and special materials to withstand the conditions required to boil acids. The combined cost of the distillation equipment and the high energy requirements make distillation a less attractive alternative for some industries. Another regeneration method involves cooling the contaminated acid to promote precipitation of the contaminants. Such a method is used by the metal pickling industry to regenerate contaminated sulfuric acid. During the metal pickling process, sulfuric acid becomes contaminated with metals and organic fluids. The common practice is to cool the sulfuric acid from its operating temperature, about 330xc2x0 K., to 273xc2x0 K. At this lower temperature, the solubility of the metals is lower and some of the contaminants precipitate out. Once the contaminants are separated from the acid, the acid bath is returned to its normal operating temperature. This process extends the life of the acid bath and minimizes the cost of neutralizing the acid prior to disposal. However, this process is not as effective in removing contaminants as the distillation processes. Thus, it would be an advantage to have a regeneration process having a greater ability to remove contaminants without involving distillation.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a method for regenerating contaminated acid without using distillation or generating solid hydrates.
It is a further object of the invention to provide an apparatus for regenerating contaminated acids.
In accordance with one aspect the invention, a method for regenerating contaminated acid is provided comprising cooling the contaminated acid to a lower temperature sufficient to form at least one liquid hydrate of the acid without forming a solid hydrate of the acid, maintaining the contaminated acid at the lower temperature for a time sufficient to form at least one liquid hydrate of the acid and precipitate an amount of the contaminants, separating the precipitated contaminants from the cooled acid, and warming the acid to decompose the liquid hydrate.
In accordance with another aspect of the invention, an apparatus is provided for regenerating contaminated acid, comprising: a tank for containing the contaminated acid, a reactor for cooling the acid to a lower temperature sufficient to form at least one liquid hydrate of the acid, the reactor having a jacket connected to a source of refrigerated coolant, a pump connected between the tank and the reactor to transfer the contaminated acid from the tank to the reactor, a separator column connected to the reactor to receive the cooled acid, the separator column having a discharge port to remove settled contaminants and an output port for a regenerated acid stream, the separator column being sized to allow sufficient time for the liquid hydrate to form.
In accordance with a further aspect of the invention, the apparatus further includes a second pump connected between the separator column and the reactor to recycle an amount of the cooled acid in the separator back into the reactor, a temperature probe attached to the output port, and a control valve connected to the output port to receive the regenerated acid stream. The control valve being responsive to a signal from the temperature probe and being capable of diverting the regenerated acid stream into separate conduits.